The large gain, power output capability, and high efficiency of GaAs FETs makes them very attractive for oscillator applications. Also, because of recent market competition, microwave GaAs FET oscillators may be lower in cost than competitive designs. However, one characteristic that has been a problem in many GaAs FET applications is the relatively large flicker noise of GaAs FETs when compared to silicon bipolar transistors and GaAs Gunn diodes. In the certain applications for which the present invention was developed, the signal-to-noise ratio of a GaAs FET oscillator is often unacceptable at lower frequencies without the present noise reduction circuit.
GaAs FET flicker noise arises from random electron capture and release in surface states and lattice defects of the GaAs crystal. Flicker noise varies as 1/f in power per unit bandwidth. With most devices, this noise would be insignificant at UHF and microwave frequencies. However, non-linearities present in any practical device lead to noise modulation when a relatively large oscillator signal is present. This noise manifests itself as amplitude modulation (AM) and frequency modulation (FM) noise exhibited as sidebands located around a carrier frequency.
The AM sidebands can be reduced by the compression characteristics of subsequent amplifiers, but the FM sidebands remain. Therefore, it is essential that the FM sidebands must be minimized to start with. The present invention lowers the sideband noise by reducing the non-linear modulation effect. Other techniques that have been used to perform this task include low frequency feedback around the device and the use of automatic gain control (AGC) instead of internal device limiting. At microwave frequencies, an AGC circuit would be difficult and expensive to implement. The circuit of the present invention, improves oscillator noise substantially, in many cases, in excess of 10 dB. Other potential applications of GaAs FET oscillators that may benefit from the present invention include devices which operate in the sub-microwave range, for example 800 Mhz.